1975 450 SL heater control
 

I have just gotten a 1975 450 SL. I guess I need some help in running the heater as I cannot understand what operates the heater control valve. The interior controls operate, except for the passenger side control the changes the heat from the floor to the side vents. What controls the heater control valve so it does not always let the heater run hot. There is a vacumn line to the control valve, however I cannot find where the line goes. As older American cars there was a cable to the control valve, there has to be something to operate this one and I just can't seem to grasp it. Anyone able to help me out here?
Thanks: confused:

The 2 outer levers MUST come all of the way down to allow micro-switches to control VACUUM control valve to pull heater valve closed.